1. Field of the Invention
The present invention relates to a method of connecting, by fusion-splicing, two optical fibers having different mode field diameter (hereinafter, referred to as MFD) from each other, and also to an optical transmission line using the two optical fibers thus spliced together.
2. Description of the Background Art
A dispersion-compensating fiber has been used to compensate chromatic dispersion of an optical transmission line comprising optical fibers. An optical amplifier having an optical fiber whose optical waveguide region is doped with a rare earth element has been used to compensate loss in the optical transmission line. In such cases, two optical fibers having different MFD have had to be connected together. Two connecting methods have been known, namely, a method of using a connector and fusion-splicing, the latter causing small connecting loss.
To connect two optical fibers by fusion-splicing, the coating on each optical fiber is removed at the adjacent end thereof, then the adjacent end faces of the two optical fibers are butted together, and the end faces are softened and fusion-spliced by heating with an arc discharge or the like. When the MFDs of the two fibers are equal to each other at the fusion-splicing portions, the splicing loss is small. When the MFDs are different, the larger is the difference in diameters, the larger becomes the splicing loss.
An optical fiber splicing method has been disclosed in Japanese Patent Application Laid Open No. 04-118607, which is intended to reduce the splicing loss when two optical fibers having different MFDs has been fusion-spliced. Two optical fibers are referred to as a first optical fiber having a larger MFD and a second optical fiber having a smaller MFD, respectively.
According to this splicing method, the MFD of the second optical fiber is enlarged by heating a portion including the adjacent end face thereof and by diffusing a dopant therein, and then the first and second optical fibers are fusion-spliced. Thus, the difference between the MFDs of the first and the second optical fibers can be reduced at the fusion-splicing portion, thereby leading to a reduction in the splicing loss.
It is an object of the present invention to provide an optical fiber splicing method for reducing splicing loss of a first optical fiber and a second optical fiber whose MFDs are different from each other, and an optical transmission line comprising such fibers.
In order to achieve this object, a method of connecting optical fibers by fusion-splicing is provided in which a first optical fiber having a first MFD and a second optical fiber having a second MFD smaller than the first MFD are connected together by fusion-splicing. The method comprises a pre-fusion heating step of heating a portion including the adjacent end face of the first optical fiber so as to diffuse a dopant, and a fusion-splicing step of connecting the first and the second optical fibers.
In one embodiment, the method further comprises a post-fusion heating step of heating a portion including the fusion-spliced part between the first and the second optical fibers after the fusion-splicing step. The dopant may be diffused until the MFD 2W1 defined by Petermann I at the adjacent end face of the first optical fiber is enlarged by at least 1 xcexcm during pre-fusion heating step. MFD 2W1 can be obtained by the following formula:             W      1      2        =                  2        ⁢                              ∫            0            x                    ⁢                                                    E                2                            ⁢                              (                r                )                                      ⁢                          r              2                        ⁢            r            ⁢                          ⅆ              r                                                            ∫          0          ∞                ⁢                                            E              2                        ⁢                          (              r              )                                ⁢          r          ⁢                      ⅆ            r                                ,
where E (r) represents the distribution of the LP01 mode.
Pursuant to another aspect of the present invention, an optical transmission line is provided in which a first optical fiber having a first MFD and a second optical fiber having a second MFD smaller than the first MFD are connected together by fusion-splicing with the above-described method.
The above and further object and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustrating only and are not intended as a definition of the limits of the invention.